The phenomenon of Surface Plasmon Resonance (SPR) can be used to detect minute changes in the refractive index at a surface of a conductive layer as some event occurs near the conductive surface, for example a metal coated surface. In particular it may be used to quantitatively determine a reaction between antigens (targets) and antibodies immobilised on the surface (probes). Surface Plasmon Resonance is due to the oscillation of free electrons which exists at a metal boundary induced by a time varying electric field absorbing photos of an incident light beam. These oscillations are affected by the refractive index of the material adjacent the metal surface and it is this that forms the basis of the sensor mechanism.
One of the most common SPR configurations involves the use of a polarized monochromatic light source (e.g. a diode laser) incident on the conductive layer, for example at the metal coated surface with a range of incident angles. In this configuration, one measures the light intensity of the reflected light beam as a function of angle over time while the event to be measured occurs, and detects a change of the angle at which a light intensity minimum occurs as a function of time.
For practical applications, SPR detection devices often include a cartridge having a liquid sample to be characterized. In many applications the cartridge is movable, in particular in some applications, the cartridge may rotate while SPR measurements are performed. SPR measurements may detect extremely small changes in a liquid sample (e.g. small concentration of a blood marker bound to corresponding probes), but may also be very sensitive to external effects (e.g. temperature, cartridge tilt and position, bulk effects). The latter can lead to a significant decrease in signal to noise ratios and so limit the application scope of SPR device.
Therefore, it would be desirable to have SPR devices providing better signal to noise ratios when compared to conventional devices. Furthermore, in the case of SPR devices including movable cartridges, it would be beneficial to have mechanisms for self-referencing and quality controls in order to ensure the quality of output data.
The specific description herein relates to a polarized double monochromatic light source incident on a detection zone. The double monochromatic light source has two wavelengths sufficiently similar so to induce the Surface Plasmon resonance effect at two similar incident angles, so that light can be captured for angles at which SPR occurs for both wavelengths, within a practical angular range. Two respective surface plasmons occur simultaneously over a predetermined critical incidence angle range and it is believed that their effect is combined to result in an increased light adsorption effect. Advantageously, the measurement of light reflection at the critical incidence angle range described herein shows a much greater sensitivity to events occurring in the liquid sample, compared with conventional known Surface Plasmon resonance devices. Furthermore, described herein are self-consistency tests associated to the double surface plasmon resonance measurements used for quality testing and feedback systems.